Light emission and the generation of photons is essential to the operation of many current and prospective technologies, in particular those which rely upon single-photon emission processes. Single-photon sources are central for many fields including quantum computing, quantum communications, quantum imaging and detection, and quantum cryptography to name a few. There are currently many forms of single-photon sources such as attenuated laser sources, quantum dots, and entangled photon sources. One type of single-photon source relies on radiative relaxation of an excited phosphor to emit a single photon at a time.
Photon sources dependent on photon emission from a phosphor are fundamentally limited by the rate at which the phosphor emits photons. The duty cycle of a phosphor-based single-photon source depends on the time it takes for a phosphor excitation to be generated and for the relaxation of the phosphor excitation to the ground state via photon emission. Generation of an excited single-photon emitting phosphor depends on the mechanism of excitation, but is typically rapid compared to the time-scale of relaxation and, consequently, photon emission. For example, when optical excitation of a phosphor is employed, excitation generation is approximately instantaneous with a speed controlled by the pulse time of the laser (e.g., tens of femtoseconds). Photon emission from an excited phosphor depends on the electronic structure of the phosphor, which is chiefly determined by the composition and structure of the material. Most phosphors have many energetically close excited electronic states that potentially emit photons with distinct properties, such photon polarization.
Many single-photon sources that rely on de-excitation of a phosphor have slow emission rates due to long relaxation times. For example, a cadmium selenide quantum dot emission time can be as much as 1 microsecond long under single-photon operating conditions. Systems that rely on single-photon sources suffer from slow photon emission rates which may result in low image resolutions and low communication and/or data rate bandwidths among other mal-effects.